let me first say that I *never* cleaned my D90, never needed to. The build in cleaner on startup/shutdown seemed to have done the trick. I'm careful about changing my lenses but certainly changed them outside when needed.

My Fuji S2 I had to clean a handful off times with Eclipse & Pec pad over the years.

Enter the D600, right off the bat the sensor was covered in crap:

Pulled out the rocket blower and gave it a good couple of blows and it looked much except for one large blotch in the bottom right corner:

Less than a week later and less than a few hundred shots and due to crunch time at work I haven't even taken it out yet, just played around the house and I count at least 4 blotches already!

Given that most of the dirt responded well to the blower, I'd surmise it is not oil. Is this sensor more prone to dust? I wonder if there's some dust floating around the mirror box since the beginning, blowing just moves it around and eventually it settles on the charged sensor again?